change D3 to peerDependencies.

[gilbarbara]

I'm using c3 in a project where I also need to have d3 installed with the latest version (3.5.5).
But using browserify to bundle my app, c3 carries its own copy of d3 (<= 3.5.0) meaning duplicate code..

cheers

[aendra-rininsland]

This isn't a bad idea at all; I use C3 in lots of projects where I also use D3. 👍

[ustccjw]

+1, and d3 version <= 3.5.0 depend jsdom(which depend contextify), but contextify can not work well in iojs (see brianmcd/contextify#173)

[aendra-rininsland]

Worth seeing the discussion in #1166 re: peerDependencies (They don't work like you think they should in newer versions of NPM, so therefore we probably won't use them as such).

Closing as a duplicate, please see #1062.